Your girlfriend isn't, by chance, a Native American Tribal member or in the Military or Military Dependent, is she?

Under the Jay Treaty, registered members of Native American Indian Tribes may cross the border with tribal-issued ID.

Valid US Military ID is also acceptable for crossing the border.

If she's not Native American or Military, then strictly TECHNICALLY legally speaking, you're out of luck.

The other posters are correct, Under the new rules imposed by the Bush Administration's Western Hemisphere Travel Initiative (WHTI), travelers must have a Passport, Passport card, Enhanced Driver's License or ID or a NEXUS Card. In general, Canada enforces the same rules because they don't want you stuck in their country.

If you can't get an enhanced ID or passport, you can always TRY assembling all the proof you possibly can of her citizenship and give it a shot. Get her Birth Certificate, high school yearbook, Drivers License, Utility Bill, Library Card, Voter Registration, A copy of her I-9 form she had to fill out to get her job, a tax return... etc etc. Get all that paperwork and head for the border. TECHNICALLY it's not enough to let her in, but they just might let it fly. Worst case scenario, you'll be turned back and can have a nice day in Bellingham.

I wouldn't worry too too much about getting stuck in Canada, if you have a stack of documents, the US Border guards will definitely hassle you, but they'll probably let you in. Worst case scenario there is you'd have to wait until Monday morning to visit the US Consulate in Vancouver.